Hylobates agilis (and H.
albibarbis)

Short phrases consisting of simple hoots, more complex
hoots ("whoo-aa") and bi-phasic hoots are uttered by males and females.
Bi-phasic hoots consist of notes alternatingly produced during exhalation and inhalation
("whoo-aa"). Some males were heard to produce relatively soft, squealing
sounds between their short phrases, similar to males of H. muelleri. Female
great call consisting of long notes of modulated frequency. A first, often very weak
climax in frequency is reached at the beginning of the great-call; a second, more
pronounced climax of higher frequency notes occurs near the end of the great call.
Male produces coda. Male solo song bouts and duet song bouts.

No clear vocal differences are known to occur between
the agile gibbon (Hylobates agilis) and the white-bearded gibbon (H. albibarbis).

Solo song, adult mated female, Louisiana Purchase Garden,
Monroe, U.S.A., Sept. 1979 (rec. Charles Welch).
This female was kept with a male (H. muelleri) and their hybird daughter.
The male of the mixed pair contributed a few squeals to the song bout. The hybrid
daughter produced several great-calls in synchrony with her mother, but not in this
example.